1. Field of the Invention
This invention relates to display electrodes for electrically driven display devices of the kind in which display electrodes are normally visible but are obscured when energized to provide a display.
2. Description of the Prior Art
Display electrodes herein described are to be found in, for example, flat panel display devices wherein the display effect is achieved by the interaction of a voltage or electric current with a medium, for example, displays using electrochromism, plating out metals from suspension, or liquid crystals. The electrodes are usually deposited on a flat substrate by evaporation, which results in a highly reflective element. Especially in passive displays, in which light is not generated by the display, electrodes with this characteristic is undesirable since the contrast between the generated display and the remainder of the display area is not good.
It should be pointed out that the treatment of aluminium in a way similar to that described hereinafter is not new. British Patent Specification No. 396,204 describes a method of producing a dense white coating on aluminum articles, for decorative purposes, by forming an adherent oxide coating and then removing the "colored constituents" without the use of external electric energy. The oxide coating may be produced by subjecting the aluminium surface to the action of a solution of an alkali carbonate and a soluble dichromate, preferably hot to obtain a coating of a greater density and thickness. Solutions of nitric acid or of nitric and sulphuric acids are preferred for removing the colored constituents. Hydrofluoric acid is preferred to treat an anodically formed oxide coating. This specification does not teach the removal of the oxide coating, and, on the contrary, states that a thick oxide coating is preferred. British Patent Specification No. 709,391 relates to a method for the surface treatment of aluminium in order to obtain a better degree of adhesion for coating, for example, lacquer. The method comprises forming a oxide layer by treatment in an aqueous solution containing caustic alkalis and soluble chromates and at least partially dissolving the oxidic layer in a second solution which is capable of dissolving trivalent chromium oxides. The specification is not concerned with the appearance of the aluminium and, like British Pat. specification No. 396,204, does not teach a way of achieving good contrast between the selected and unselected electrodes of an electrically driven display device.